Hinduism and Hindutva are completely different terms. Hinduism is pro religious harmony while Hindutva is pro religious extremism that ensures never ending hatred and division among society.

Hinduism -> Hindu Religion

Hindutva -> Hinduness (Coined by VD Savarkar).

Hindutva fascists talk about a Nation based on Hindu religion, "Hindu Rashtra". Therefore the fanatics fantasizing and working towards achieving the goal of Hindu Nation by creating divisions by spreading hate among the populous, practicing religious extremism(hindutva) are known as Hindu Nationalist.